Protective coating for aluminum and its alloys



Patented Sept. 5,

admits PROTECTIVE ATING FORALUMINUM AND rrs armors .Iunius 1). Edwards, Oakm'ont, and Ralph B,

Mason, ,New Remington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application June 12,, 1986, Serial No. 84,900 r 1 Claims. (CI. 91-00) This invention relates generally to providing metallic surfaces with protective coatings, and in particular to the provision of adherent protective films. such as paint films, on aluminum and 5 aluminum base alloys. v

It has been known to prepare metallic surfaces, prior to an application of paint thereto, by roughening, as by means of sand blasting or other means of mechanical abrasion. It has also been known to employ chemical etching solutions for the purpose of roughening metallic surfaces prior to the application of a paint film thereto.

For certain services, however, where the coating must protect against corrosive attack and also 5 be resistant to mechanical shock and abrasion.

these methods of coating have proven inadequate to prevent chipping or peeling of the coating with attack by the corrosive mediums to which the painted metal or article made therefrom has been exposed. 1

It is an object of the present invention to provide an adherent, corrosion-resistantrfilm of paint, varnish or lacquer upon the surface of aluminum and aluminum base alloys which will withstand severe chemical attack as well as mechanical shock and abrasion.

A further object of the invention is to provide a method of treating aluminum and aluminum base alloys for the purpose of providing a surface capable of retaining a protective film, such as a paint or varnish film, which method is economi cal and commercially adaptable to numerous applications. o

Other objects will present themselves on consideration of the following description of the invention, in which specific examples of the invention are illustrated.

The invention has .been found'to be particularly useful in the manufacture and preparation so of equipment for use in the manufacture of rayon. Aluminum and by the term "aluminum" as used herein and in the appended claims is meant aluminum and aluminum base alloys is receiving increasing attention and use in the rayon industry, but the applications of this metal are, to some extent, curtailed because certain of the solutions employed in the manufacture of rayon are corrosive to aluminum. However, it has been discovered that equipment made from aluminum. when treated in accordance with the invention to be hereinafter described, has successfully withstood the corrosive attack of both the acid and the alkaline solutions, as well as the mechanical shocks of handling to which such on equipment is subjected in the rayon industry.

The invention essentially comprises 'a multistep process, in which the aluminum article or surface to be provided with a protective film of paint or the like is first thoroughly cleaned of any grease or oxide thereon in a. suitable cleaner, ii such as a solution of an alkali. The second step in the process comprises. a treatment of the aforesaid article or surface in a solution of a heavy metal andan acid to produce the particular type of etch which is one of the salient features of the present invention. The third and last step of the invention, prior to the actual application of the paint film or coating, com,- prises a neutralizing and/or cleaning step whereby the metal surface is relieved and freed from objectionable contaminating substanm which would otherwise be entrapped beneath the paint film. This last step in the invention is preferably a treatment in an acidic solution and is of particular importance to the success of the in- 2 vention, since it has been found that a surface a prepared in accordance with the first two steps or treatments only and thereafter painted does not protect against chemical attack in the same manner as do surfaces treated in accordance with 25 7 this invention.

In the commercial practice of the invention, the initial cleaning step is preferably conducted with an alkaline reagent such as sodium hydroxide, although any other suitable cleaning J0 treatment such as with hydrofluoric acid, is permissible, providing the natural metal oxide film, or that formed by heating during the fabrication of the aluminum, together with other foreign material such as grease or the like, lslfi removed. The object of this preliminary cleaning is to permit uniform attack of the metal by the following etching treatment. In this connection, it will be manifest 'that the cleaning step of this invention is not always essential, this lo cleaning step depending entirely upon the surface condition of the article to be provided with a paint film or protective coating. Where a cleaning step is desired and necessary, it has been found that a solution of sodium chloride or 45 sodium fluoride with sodium hydroxide, or various combinations thereof are exceptionally satisfactory. Also, mineral spirits and other similar cleaners have likewise been satisfactory whiz-e it is only'necessary to remove grease or 50 o r surfacelubricants which have accumulated on the surface of the article to be coated during its process of manufacture.

, A very. satisfactory cleaning solution contains from about 1.0 per cent to 10.0 per cent sodium is Iii) chloride and from about 2.0 per cent of 20.0 per cent sodium hydroxide. Preferably this solution should be maintained between about room temperature and 200 F.,depending upon the period of time for,which the article or surface is to be immersed therein. For example, at lower temperatures it will be necessary to maintain the article immersed for a longer period than at an elevated temperature, but two minutes at 170 to 180 F., followed by a rinse in hot water, has been found commercially satisfactory in a solution containing 5.0 per cent sodium chloride and 5.0 per cent sodium hydroxide. Weaker or stronger solutions will act slower or faster accordingly.

Having subjected the article to the initial cleaning treatment aioredescribed, the second treatment of the invention comprises the etching step. The etching solution, which has been found essential in this step of the invention, contains hydrochloric acid and a heavy metal. The heavy metals which have been found to be satisfactory are manganese, iron, copper, nickel, or cobalt. These metals are conveniently introduced into the solution as salts, such as the sulphate or chloride, for example. A suitable etching solution for this second treatment or step in the process preferably comprises from about 0.5 per cent to 5.0 per cent manganese sulphate 0r chloride (equivalent to about 0.1 to 1.0 per cent manganese) in a solution made by diluting one volume The iron can be used in the hydrochloric acid solution in about the same concentration as the manganese. Copper, nickel or cobalt accelerate the etching action greatly and are preferably used in the range of about 0.02 to 0.1 per cent. Greater or lesser amounts of the specified heavy metals may be used in the etching solution, but there is usually no advantage in doing so. The concentration of the hydrochloric acid may also be varied, depending upon the rate of reaction desired. With 0.025 per cent of copper, a solution of one part of concentrated hydrochloric acid in nine parts of water has given good results. The time of immersion will depend upon the rate of reaction with the metal. The time required de creases with increase in temperature or the etching solution; a temperature of about 100 F. has

been found to be convenient one at which to operate. The action will vary somewhat with different alloys, but the correct time to secure the desired etching can readily be determined by a preliminary test.

The etching step may be followed by a clean metal, so that cleaning in cold water is all that is necessary.

The third and final step of the process, prior to the application of the paint film, has been successfully carried out in a chromic acid solution. The term "chromic acid solution" is meant 'to include a solution of a dichromate, chromic acid, or combinations thereof.

In actual practice a satisfactory solution is one containing the equivalent of about 1.0 per cent to 10.0 per cent chromic anhydride. An immersion in a chromic acid solution equivalent to 5.0 per cent of chromic anhydride for minutes at 180 F., followed by a cold water rinse, relieves the previously treated article of residual chlorides and contaminating substances which might induce corrosion beneath the later applied paint film.

The article subjected to the three treatments above described, or subjected to the second and third treatments where the initial cleaning step is not required, should be thoroughly dried prior to the application of a paint film to the surface thereof. Synthetic resin coatings, particularly those of the phenol aldehyde condensation type, have been found highly satisfactory. -'They may be employed with or without pigments, and the term "paint", as used in this specification, covers broadly protective coatings, whether they are termed paints, varnishes, or lacquers in common parlance. A tenacious bond is established between the surfaces so treated and the paint so that equipment so treated has satisfactorily withstood the acid and alkaline corrosive solutions employed in the rayon industry, as well as the severe mechanical treatment to which such equipment is subjected in use.

In the production of forged aluminum alloy rayon spinning buckets in an actual plant installation, the forged buckets were first treated in a cleaning solution containing sodium hydroxide and sodium chloride made up as follows:

. Grams Chemically pure sodium hydroxide 100 Commercial sodium chloride 100 Diluted to 2000 cc. with water.

Concentrated hydrochloric acid solution---" volume 1 Water volumes 2 Manganese sulphate -grams per liter 4 The buckets were immersed in this solution for about three minutes, the solution having been held at about 90 F. The buckets were then subjected to a cold water rinse.

Following the above two treatments, the buckets were immersed ina chromic acid solution containing 5.0 per cent of chromic anhydride in water. The buckets were maintained in this solution, which was held at about 185 F., for 15 minutes, after which they were thoroughly rinsed in cold water.

The buckets were thereafter thoroughly dried and a Bakelite varnish coating applied thereto.

Although the invention, has been describedin connection with a specific application of a multistep process for the preparation of aluminum and aluminum base alloy surfaces prior to the application of paint thereto, it is to be understood that the invention is not to be limited in any way except as defined in the claims appended hereto.

What we claim is:

l. The method of providing a protective paint film on an aluminum surface, comprising etching said surface with a hydrochloric acid solution containing a heavy metal selected from the group consisting of manganese, iron, copper, nickel, and cobalt, treating the etched surface with a chromic acid solution, and thereafter applying a paint 2. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 per cent to 1.0 per cent of a metal selected from the group of metals consisting of manganese, iron, copper, nickel, and cobalt, treating the etched surface with a chromic acid solution, and thereafter applying a paint film. I

3. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 per cent to 1.0 per cent of a metal selected from the group of metals consisting of manganese, iron, copper, nickel, and

.ing the paint, surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of copper, washing the etched surface with a nitric acid solution to remove any superficial copper deposited during the etching treatment, treating the etched surface with a chromic acid solution, and thereafter applying a paint film.

5. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of nickel, washing the etched surface with a nitric acid solution to remove any superficial nickel deposited during the etching treatment, treating the etched surface with a chromic acid solution, and thereafter applying a paint film. 6. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of cobalt, washing the etched surface with a nitric acid solution to remove any superficial cobalt deposited during the etching treatment, treating the etched surface with=a chromic acid solution, and thereafter applying a paint film.

7. The method of providing a protective paint film on an aluminum surface comprising etching said surface with a hydrochloric acid solution containing a heavy metal selected from the group consisting of manganese, iron, copper, nickel, and ,cobalt, washing the etched surface to remove any superficial heavy metal deposited during the etching treatment and then treating the said surface with a chromic acid solution and thereafter applying a paint film.

JUNIUB D. EDWARDS. RALPH B. MASON.

FERIIFIGATE OF CORRECTION.

Patent no. 2,171, 515.

September 5, 1939.

1 J'UHIU3 DY. EIMARDS, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 61, before the word "convenient" insert a; pags3, f1rst column, line 53, claims i strike out the word more, and me the said Lettors Patent should be read with 1211s correction therein that the same may conform to the record of the case in the Patent Office.

Signed "on sealed can ea, of 0mm, 1; n. 19 9.

Henry Van Arsdale, Acting commissioner of Patents.

the invention is not to be limited in any way except as defined in the claims appended hereto.

What we claim is:

l. The method of providing a protective paint film on an aluminum surface, comprising etching said surface with a hydrochloric acid solution containing a heavy metal selected from the group consisting of manganese, iron, copper, nickel, and cobalt, treating the etched surface with a chromic acid solution, and thereafter applying a paint 2. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 per cent to 1.0 per cent of a metal selected from the group of metals consisting of manganese, iron, copper, nickel, and cobalt, treating the etched surface with a chromic acid solution, and thereafter applying a paint film. I

3. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 per cent to 1.0 per cent of a metal selected from the group of metals consisting of manganese, iron, copper, nickel, and

.ing the paint, surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of copper, washing the etched surface with a nitric acid solution to remove any superficial copper deposited during the etching treatment, treating the etched surface with a chromic acid solution, and thereafter applying a paint film.

5. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of nickel, washing the etched surface with a nitric acid solution to remove any superficial nickel deposited during the etching treatment, treating the etched surface with a chromic acid solution, and thereafter applying a paint film. 6. The method of providing a protective paint film on an aluminum surface, comprising etching the surface with a hydrochloric acid solution containing from about 0.02 to 0.1 per cent of cobalt, washing the etched surface with a nitric acid solution to remove any superficial cobalt deposited during the etching treatment, treating the etched surface with=a chromic acid solution, and thereafter applying a paint film.

7. The method of providing a protective paint film on an aluminum surface comprising etching said surface with a hydrochloric acid solution containing a heavy metal selected from the group consisting of manganese, iron, copper, nickel, and ,cobalt, washing the etched surface to remove any superficial heavy metal deposited during the etching treatment and then treating the said surface with a chromic acid solution and thereafter applying a paint film.

JUNIUB D. EDWARDS. RALPH B. MASON.

FERIIFIGATE OF CORRECTION.

Patent no. 2,171, 515.

September 5, 1939.

1 J'UHIU3 DY. EIMARDS, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 61, before the word "convenient" insert a; pags3, f1rst column, line 53, claims i strike out the word more, and me the said Lettors Patent should be read with 1211s correction therein that the same may conform to the record of the case in the Patent Office.

Signed "on sealed can ea, of 0mm, 1; n. 19 9.

Henry Van Arsdale, Acting commissioner of Patents. 

